1. Field of the Invention
The present invention relates to lead-free optical glass materials which have refractive indices nd of between 1.54 and 1.61 and Abbe numbers xcexdd of between 38 and 45. Glass materials of this type belong to the optical glass family of the light flint glass materials.
2. Prior Art
Because the glass ingredients PbO and As2O3 have become the subject of public discussions regarding environmental pollution which they cause, the manufacturers of optical equipment need PbO-free and preferably also As2O3-free glass materials having the appropriate optical properties.
Simply exchanging the lead oxide for one or more ingredients does not generally allow the PbO-influenced, desired optical and glass technology properties to be reproduced. Instead, new developments or far-reaching changes in the glass composition are required.
Lead-free glass materials with similar optical characteristics and a similar composition are already known.
The closest prior art glass materials are described in JP 56-59640 A. However, the glass materials for spectacles described in that document contain only 11 to 18% by weight alkali metal oxides, which may be composed of Na2O, K2O and Li2O. Furthermore, they optionally contain the ingredients Nb2O5, La2O3 and also ZrO2, which in this case inhibit crystallization but are expensive, as partial replacements for TiO2.
It is an object of the present invention to provide a lead-free optical glass material with a refractive index nd of between 1.54 and 1.61 and an Abbe number xcexdd of between 38 and 45 which can be produced at low cost and have good melting and processing properties. This also includes the requirement for sufficient crystallization stability.
According to the invention the lead-free optical glass material with a refractive index nd of between 1.54 and 1.61 and an Abbe number xcexdd of between 38 and 45 has a composition, based on oxide content, of:
and at least one refining agent, if needed, in an amount suitable for the purposes of refining; with the proviso that a sum total of Na2O and K2O present is greater than 18% by weight.
The glass materials according to the invention contain the glass formers SiO2 and Al2O3, in balanced proportions of 52 to 62% by weight SiO2 and 3 to 8% by weight Al2O3. In this way, both the meltability of the glass materials, which deteriorates as the Al2O3 content rises, and their chemical durability, which would deteriorate if the Al2O3 content were too low, are satisfactory. Moreover, if the above-mentioned minimum Al2O3 content were to be increased, the risk of devitrification would rise excessively. There is no B2O3, which is a common third glass former, used, which is of benefit to the chemical durability of the glass materials. An SiO2 content of between 53 and 59% by weight is preferred, but an SiO2 content of between 55 and 59% is particularly preferred. The Al2O3 content is preferably between 4 and 7% by weight, particularly preferably between 5 and 7% by weight.
To obtain the desired high refractive index combined, at the same time, with a low Abbe number, the glass materials contain relatively large amounts of TiO2, at 13 to 18% by weight. The TiO2 content is preferably between 14 and 17% by weight, particularly preferably between 15 and 17% by weight. In addition to TiO2, the glass materials according to the invention may also contain up to 5% by weight ZrO2. The presence of the two ingredients improves the chemical durability, in particular the alkali resistance. If the levels were to be higher, the crystallization stability would be considerably reduced. Preferably, the sum of the TiO2 and ZrO2 is less than 18% by weight. In preferred embodiments, there is no ZrO2.
To improve the meltability, that is to reduce the melting temperatures, with the given high level of glass formers, the glass materials contain from 7 to 14% by weight Na2O and 8 to 14% by weight K2O as fluxing agents. If the levels were to be higher, in particular in relation to the high TiO2 content, the tendency towards crystallization would rise excessively. Therefore, the sum of R2O and TiO2 is preferably less than or equal to 42% by weight. For the same reason, LiO2 is dispensed with altogether. Increasing the alkali metal content further would also mean that the desired range of refractive indices would not be reached. The sum of Na2O and K2O is at least greater than 18% by weight. An Na2O content of 8 to 13% by weight and a K2O content of 10 to 14% by weight are preferred. An Na2O content of 8 to 11% by weight and a K2O content 10 to 12% by weight are particular preferred.
To improve the quality of the glass, one or more refining agents, which are known per se, may be added in the customary amounts to the batch in order to refine the glass material. This provides the glass material with a particularly good internal glass quality with regard to the freedom from bubbles and cords.
If the refining agent used is not As2O3, but rather, instead, Sb2O3, for example, which is possible without any loss in glass quality, the lead-free glass materials according to the invention are additionally free of arsenic.
The Sb2O3 content is preferably between 0.1 and 0.5% by weight. It is also preferable for the glass materials, if appropriate in addition to Sb2O3, to contain up to 0.5% by weight fluoride, which likewise has a refining action. Fluoride is added, for example, as NaF.
In addition to a refractive index nd of between 1.54 and 1.61, and an Abbe number of between 38 and 45, and a positive anomalous partial dispersion in the blue region of the spectrum, the glass materials according to the invention have the benefits as described in the following paragraphs.
The glass materials are PbO-free and, in a preferred embodiment are also As2O3-free. The glass materials exhibit good crystallization stability. This allows production in a continuously operating melting unit. A measure of the crystallization stability being sufficient for production of this nature is the viscosity at the liquidus temperature. For continuous production, it should be xe2x89xa71000 dPas. This condition is satisfied with the glass materials according to the invention. Since the glass materials exhibit a crystallization stability of this nature, further thermal treatment of the glass materials, such as pressing or repressing, also becomes possible. It is advantageous that it is possible to dispense with the expensive ingredients Nb2O5 and La2O3 while the glass materials, despite the high TiO2 content, still exhibit a sufficient crystallization stability, this fact also being contributed to by the fact that alkaline-earth metal oxides are eliminated altogether. Therefore, apart from inevitable impurities, the glass materials are free of alkaline-earth metal oxides, Nb2O5 and La2O5.
In addition, the xe2x80x9clength of the glass materialsxe2x80x9d (viscosity curve which falls slowly with temperature) ensures good processability despite the relatively high melting temperatures of approximately 1400xc2x0 C.
The glass materials have an exceptional alkali resistance, as attested to by the fact that they belong to the alkali resistance class AR=1, while also retaining a chemical durability which is adequate in other respects. The chemical durability of the glass materials is of importance for their further processing, such as grinding and polishing.
Glass materials within a compositional range (in % by weight based on oxides) SiO2 from 52 to 62, preferably from 53 to 59, % by weight; Al2O3 from 3 to 8, preferably from 4 to 7, % by weight; Na2O, from 7 to 14% by weight, K2O from 8 to 14% by weight, with a sum of Na2O and K2O greater than 18% by weight; preferably Na2O from 8 to 13% by weight and K2O from 10 to 14% by weight; TiO2 from 13 to 18, preferably from 14 to 17, % by weight; ZrO2 from 0 to 5% by weight, with a sum total of TiO2 and ZrO2 present is not greater than 18% by weight, preferably ZrO2-free; have a refractive index nd of between 1.55 and 1.60 and an Abbe number xcexdd of between 39 and 44. Glass materials of composition (% by weight based on oxides present) SiO2 from 55 to 59% by weight; Al2O3 from 5 to 7% by weight; Na2O, from 8 to 11% by weight; K2O, from 10 to 12% by weight, with a sum of Na2O+K2O greater than 18% by weight, TiO2 from 15 to 17% by weight have a refractive index nd of between 1.56 and 1.59 and an Abbe number xcexdd of between 39 and 44.